Project Summary/Abstract The stability of the mammalian genome depends on a remarkable toolkit of surveillance, repair, signaling, and checkpoint mechanisms. Mutations in the DNA itself, or corruptions in many of these genome integrity mechanisms, can result in disease including cancer. Given the importance of mutation and cell division in tumorigenesis, two central pathways in genome integrity are the DNA damage response, and the cell division cycle. A more complete understanding of these pathways is crucial to our knowledge of normal cellular development in homeostasis, stem cell biology, the etiology of cancers, as well as for technical applications like gene targeting. This project seeks to tackle outstanding problems in these fields in order to elucidate fundamental molecular cell biology mechanisms that can improve therapeutic outcomes in cancer. The F99 phase is focused on double-strand break (DSB) repair and the control of end resection, a critical molecular ?choice? of whether to repair a DSB by blunt end-joining or by homologous recombination. I revealed novel mechanistic insights about the protein controlling this choice, 53BP1, findings relevant to the treatment of BRCA1-deficient cancers with PARP1 inhibitors. We found that?instead of blocking end resection as was generally thought?53BP1 recruits polymerase alpha to counteract resection by fill-in synthesis of the resected DSB. In the remainder of the dissertation work, I will explore how BRCA1 and 53BP1 regulate resection and fill-in synthesis in light of this new model. I will also gain the necessary experience and exposure scientifically and professionally to transition to a cancer-focused postdoc in a stellar lab. For the K00 phase, I will shift my focus and approaches to study mechanisms preserving genome integrity in the critical window of mitosis, where diverse chromatin biology pathways converge. I plan to learn and implement high throughput screens, computational analysis of larger, statistically-powerful data sets, as well as in vivo modeling in the mouse, and analysis of sequencing data from human tumor samples. These new approaches, coupled with my already strong background in genetics, microscopy, and biochemistry, will allow me to address the most pressing and challenging issues in genome integrity and cancer biology today. With the aid of this award, I intend to continue my research contribution and gain experience in order to become a leader of my own cancer-focused lab and a leader in the field of genome integrity.